The present invention concerns an optical transmitter having a modulation-capable wavelength-stable laser source. Patent 2 308 461.
European Patent 0 584 647 describes an optical transmitter which is used in optical telecommunications. The transmitter is composed of a laser source, whose signal is modulated into the transmission link before the coupling. A transmitter of this type is used in broadband communications to transmit data over great distances. The transmission system, in this context, operates on a given wavelength. The transmitter is unprotected against interference that arises from light of other wavelengths and that can lead to the output signal being subjected to distortion and noise.
The optical transmitter according to the present invention has the advantage that only the optical output at the transmission wavelength is coupled by reflection into the transmission link via a Michelson interferometer, in a wavelength-selective manner. Wavelengths outside of the bandwidth of the Michelson interferometer pass through it and can be absorbed (drained off). The band-pass filter prevents signals of wavelengths other than the wavelength of the transmitter from arriving over the coupled fiber-optic lines into the transmitter. In this manner, the laser source remains free of interference and harmonics of different wavelengths, which leads to very stable wavelength selection. In this manner, it is possible to use the transmitter according to the invention in a system that employs a wavelength multiplex as its transmission method. For use in a wavelength multiplex transmission system, it is not necessary to take any further.
As a result of the measures indicated in the subclaims, an advantageous refinement and improvement of the optical transmitter described in the main claim is possible.
It is particularly advantageous if the laser source is composed of a semiconductor laser, whose field distribution at the front end is broadened into a coupled strip waveguide. Due to the beam expansion, the semiconductor laser can be coupled to the strip waveguide passively and therefore in a simple manner. To adjust the wavelength, an antireflective layer is applied to the end surface, which is coupled to the strip waveguide, the antireflective layer eliminating the laser resonator of the semiconductor chip. The coupled strip waveguide is composed of a silica-glass-core step-index structure, the waveguide core being made of glass doped using germanium. A Bragg grating can be written into the waveguide core using UV light. The wavelength-selective Bragg grating and the other semiconductor laser end surface, facing away, form the laser resonator. As a function of the wavelength selection of the Bragg grating, the laser oscillates on the wavelength of the Bragg grating. The data signal of the optical output can be modulated via the internal current modulation of the laser diode. It is advantageous if both the strip waveguide of the laser diode as well as the glass strip waveguide are executed so as to be diagonal with respect to the antireflective layer. In this manner, residual reflections at a non-ideal antireflective layer are suppressed both in the direction of the semiconductor laser as well as of the Bragg grating. The specific reflected output is not coupled into the respective strip waveguide on account of the canted end surface. In the emission spectrum, no additional mode structure that could lead to mode jumps are formed in the antireflective layer as a result of the suppression of the reflection.
Advantageously, another version of a wavelength selective transmission source can also be used. The highly stable wavelength selective source is designed as an erbium/ytterbium-doped glass waveguide DFB laser. A semiconductor laser is used as the pump source. The optical output power can be adjusted via the magnitude of the pumping power. The modulation takes place externally, which has the advantage that higher modulation rates are possible, extending into the gigabyte range.
Advantageously, all transmitters coupled to the transmission optical fiber by reflection have a wave absorber for those wavelengths that are not emitted by the laser diode. This wave absorber can be composed of bevelings or a raw edge of the planner glass strip waveguides ends that are not coupled into the laser diode and the transmission optical fiber.